Generally air displacement type pumps include both an inlet valve and an outlet valve that are suitable controlled by some form of timer system, see for example U.S. Pat. No. 2,383,193 issued Aug. 21, 1945 to Hiebert or by means of weight sensors or other types of controls to actuate the valve at the appropriate time as required by the pumping process.
In systems where self-actuating valves are used, i.e. flap valves that are moved from open to closed position depending on the pressure differential across the valve, the valve is open or closed depending on the control to the pump, i.e. the application of positive or negative pressure to the main pump chamber which is controlled for example as described in U.S. Pat. No. 2,943,578 issued Jul. 5, 1960 to McCombie.
Air displacement type pumps have found to be particularly suitable in pumping frail materials such as fish slurries, etc. and preventing damage to the fish. A recently issued U.S. Pat. No. 4,770,610 issued Sept. 13, 1988 to Breckner, teaches the use of bladder type valves on inlet and outlet lines to an air dispacement pump used for pumping fish. In this system, the bladder valves are opened and closed by applying pressure to the outside chamber of the bladder valve to close the valve at the appropriate time. The timing of this opening or closing of the valve is preferably obtained by means of computer operated solenoid valves controller to operate in the desired sequence for opening and closing of the inlet and valves and applying positive and negative pressure to the main pump chamber.